Ni-NiO core-shell inverse opal electrodes for supercapacitors

Article abstract of DOI:10.1039/c0cc05191h

A general template-assisted electrochemical approach was used to synthesize three-dimensional ordered Ni core-NiO shell inverse opals (IOs) as electrodes for supercapacitors. The Ni-NiO IO electrodes displayed pseudo-capacitor behavior, good rate capability and cycling performance.

Full text of DOI:10.1039/c0cc05191h

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Ni–NiO core–shell inverse opal electrodes for supercapacitorsw
Jae-Hun Kim, Soon Hyung Kang, Kai Zhu,* Jin Young Kim, Nathan R. Neale and
Arthur J. Frank*
Received 26th November 2010, Accepted 2nd March 2011
DOI: 10.1039/c0cc05191h
A general template-assisted electrochemical approach was used
to synthesize three-dimensional ordered Ni core–NiO shell
inverse opals (IOs) as electrodes for supercapacitors. The
Ni–NiO IO electrodes displayed pseudo-capacitor behavior,
good rate capability and cycling performance.
(NP) electrode. Secondly, the periodic pore structure inherent in
ordered inverse opals should enable better ionic conduction in the
electrolyte filled pores than the tortuous pore structure in
disordered metal oxide NP electrodes.
In this study, we describe the synthesis of a Ni–NiO core–shell
structure using electrodeposition of Ni metal within a polystyrene
bead template and subsequent electrodeposition and thermal
annealing to form a NiO shell. The electrochemical characteristics
of the resulting Ni–NiO core–shell IO architecture are then
examined as a new design for supercapacitor electrodes.
The multilayer polystyrene (PS) opal template was prepared by
slow gravimetrical sedimentation of colloidal particles onto a
Electrochemical capacitors (also called supercapacitors and ultra-
capacitors) are emerging energy storage devices because of their
high power density, long cycle life, short charging time, and
1–3
minimal safety concerns. Such devices can be used either by
themselves as a primary power storage source or as an auxiliary
power storage source with rechargeable batteries to meet the
energy needs for high power applications such as electric vehicles
and industrial mobile equipment. Pseudo-capacitors (or redox-
capacitors) represent one type of supercapacitors that have the
potential to achieve high energy densities resulting from the fast
and reversible redox reactions at/near the interface of the active
storage material and an electrolyte. Among various pseudo-
1
7
substrate as detailed elsewhere. Polystyrene colloid spheres
(Seradyn, carboxylate-modified, medium acid) with an average
diameter of 400 nm were deposited onto fluorine-doped tin oxide
(FTO) substrates (Hartford Glass, TEC 15). Electrodeposition
was carried out using a potentiostat/galvanostat (VMC-4,
Princeton Applied Research) in a three-electrode cell using the
PS coated substrate as the working electrode, a platinum mesh
counter electrode, and an Ag/AgCl reference electrode. To obtain
Ni–NiO inverse opals, Ni metal (ca. 460 mg) was first electro-
capacitor materials, hydrous RuO exhibits the highest perfor-
2
4,5
mance to date, but its high cost limits its commercial use.
Therefore, there has been considerable effort in exploring alter-
native electrode materials showing pseudo-capacitive behaviors,
deposited within the pore space of the PS films in a bath prepared
À1
6
,7
8
9–16
such as manganese oxide, cobalt oxide, and nickel oxide.
by dissolving a mixture of 0.91 M (240 g L ) NiSO
À1
4
Á6H
2
O, 0.13 M
BO in
À1
For high-power applications, adequate power density is only
achieved with fast electron and ion transport. Conventional
nanostructured electrode materials usually consist of a disordered
porous matrix (e.g., sintered nanoparticle film); these electrode
materials exhibit poor electrical and mechanical contact and an
ill-defined pore network that impedes electronic and ionic
conductions during charge/discharge cycling. This study addresses
these challenges simultaneously with a structural motif that has
not been applied to supercapacitors: a metal core–metal oxide
shell inverse opal (IO) electrode. The electrode architecture could
potentially improve power density in two ways. The Ni metal core
is expected to result in more facile electrical conductivity compared
with that of the conventional disordered metal oxide nanoparticle
(30 g L ) NiCl
2
Á6H
2
O, and 0.49 M (30 g L ) H
3
3
distilled water solution at a constant potential of À1.0 V (vs. Ag/
AgCl) for 12 minutes. Samples were then immersed in dimethyl-
formamide for 3 h at 120 1C to dissolve the PS template (thickness:
about 2 mm). To prepare the thin NiO active material on the Ni
11
inverse opals, galvanostatic electrodeposition of nickel hydroxide
was performed in 0.1 M Ni(NO solution at a current density of
3 2
)
À2
1 mA cm for 5 minutes followed by annealing at 300 1C in air
for 1 h; approximately 100 mg NiO was deposited on the surface of
the Ni IO structure. Nickel oxide nanoparticles (NP) were
purchased from Aldrich (99.8%, o50 nm, specific surface area
2
À1
of >50 m g by BET). Films of the NiO NPs were prepared by
18
a similar procedure described for preparing TiO NP films.
2
Briefly, a single-layered NiO film was applied from an organic
paste (18.2 wt% NiO, 9.1 wt% ethyl cellulose, 72.7 wt% terpineol)
via the doctor blade technique, and then it was annealed at a
temperature of 300 1C. The average thickness of the NiO NP films
was about 2 mm.
The crystallinity of the samples was determined by X-ray
diffraction (XRD, SCINTAG DMS-2000 diffractometer with
Cu Ka radiation). The morphology and microstructure of the
samples were examined by field emission scanning electron
Chemical and Materials Science Center, National Renewable Energy
Laboratory, Golden, CO 80401, USA. E-mail: Kai.Zhu@nrel.gov,
AFrank@nrel.gov; Fax: +1 303 384 6150;
Tel: +1 303 384 6353, 6262
w Electronic supplementary information (ESI) available: Current–time
profile for Ni deposition, voltage–time profile for Ni(OH)
2
deposition,
À2
voltage profiles of the Ni–NiO inverse opal electrode cycled at 1 mA cm
and the cyclic voltammograms of the NiO nanoparticle electrode at scan
rates from 5 to 500 mV s . See DOI: 10.1039/c0cc05191h
À1
5
214 Chem. Commun., 2011, 47, 5214–5216
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Fig. 2 XRD of (a) Ni metal inverse opals and (b) Ni–NiO inverse
opals annealed at 300 1C. The peaks indicated by asterisks correspond
to the FTO substrate.
Fig. 1 SEM images of (a) polystyrene beads, (b) Ni metal inverse
opal structure, (c) Ni–NiO inverse opal structure, and (d) TEM image
of Ni–NiO inverse opal structure showing the (110) plane.
microscopy (FE-SEM, JEOL JSM-7000F) and high-resolution
transmission electron microscopy (HR-TEM, FEI Tecnai
F-20UT, operating at 200 kV). Electrochemical measurements
were performed with a three-electrode glass cell set-up,
consisting of a Ni–NiO IO working electrode, a platinum
mesh counter electrode, and an Ag/AgCl reference electrode.
Galvanostatic charge/discharge cycling and cyclic voltammetry
were carried out using the potentiostat/galvanostat from 0.0 to
0.5 V in 1 M KOH solution at room temperature.
Fig. 1 displays SEM images of the (a) PS opals, (b) Ni metal
inverse opals (IOs), (c) Ni–NiO (thin NiO film on a Ni metal core)
IOs, and (d) TEM image of Ni–NiO IOs. It can be seen that the
PS opals (Fig. 1a) are well-ordered with 400 nm diameter particles
in hexagonal symmetry (face-centered cubic lattice: ABCABC).
Most samples exhibit some cracks and vacancies which result
19
from film shrinkage during evaporative self-assembly. The
electrodeposition of Ni metal with subsequent removal of the
PS template produces a well-ordered Ni IO structure (Fig. 1b).
Electrodeposition of nickel hydroxide and subsequent annealing
leads to a roughening of the IO surface, but otherwise retention of
the macroscopic pore ordering (Fig. 1c). Analyses of the SEM
images of Ni IO (Fig. 1b) and Ni–NiO IO (Fig. 1c) structures
show that the average NiO coating thickness is 12 Æ 4 nm.
Finally, TEM revealed that the dark Ni metal core network is well
connected, suggesting that the core should provide a continuous
electrically conductive pathway (Fig. 1d).
Fig. 3 (a) Cyclic voltammograms of the Ni–NiO inverse opal
À1
electrode annealed at 300 1C at various scan rates (5–500 mV s
)
and (b) capacitance of the Ni–NiO inverse opal and nanoparticle
electrodes as a function of the scan rate.
peaks are associated with the pseudo-capacitive behavior
resulting from reversible faradic redox reactions:
Fig. 2 shows the XRD patterns of inverse opals of (a) Ni
metal and (b) Ni–NiO structure annealed at 300 1C. The
characteristic peaks of rock salt NiO at 37.31 (111), 43.31
(200), 62.91 (220) were observed in the XRD patterns after
electrodeposition and subsequent annealing. Annealing
À
À
NiO + OH 2 NiOOH + e
(1)
The CV curves in Fig. 3a are quite symmetric, implying good
reversibility of the redox reactions (eqn (1)) on the NiO
surface. The shape of the current responses are essentially
the same over the entire range of scan rates, indicative of rapid
faradic reactions at high scan rates. The current response
increases approximately proportionally with increasing scan
rates, suggesting that the rates of electronic and ionic transport
transforms the as-deposited nickel hydroxide phase to crystalline
11
NiO.
Fig. 3a presents the cyclic voltammograms of the Ni–NiO
IO electrode annealed at 300 1C in the potential window of 0.0
to 0.5 V. Pairs of well-defined cathodic and anodic peaks are
clearly observed over the entire range of scan rates from 5 to
À1
11–14
À1
5
00 mV s . Studies
of NiO electrodes indicate that these
are not limiting at scan rates as high as 500 mV s . Notably,
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5214–5216 5215

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a Ni metal IO annealed without the electrochemically-deposited
Ni(OH) film showed 3-fold less capacitance than that of the
capacitance versus cycle number, is presented in Fig. 4b. The
capacitance of the Ni–NiO IO electrode increases slightly with
cycling, which could be ascribed to a slow infiltration of
electrolyte into the nanopores within the NiO layer during
the cycling process. The weight of the NiO active material was
estimated from the structural parameters of the IO films. From
the FCC lattice, pore size (ca. 300 nm), and NiO shell
layer thickness (ca. 12 Æ 4 nm), we estimate that the specific
capacitance of the Ni–NiO IO electrode was about
2
NiO deposited film, suggesting that the electrochemically-
prepared NiO has stronger capacitive behavior than that from
oxidized Ni metal. Fig. 3b shows the scan-rate dependence of
the capacitance of the Ni–NiO IO electrode. The rate
capability of a NiO NP film with a similar film thickness is
also plotted in Fig. 3b for comparison. The IO electrode
exhibits a higher rate capability than the NP electrode. At
À1
À1
the highest scan rate (500 mV s ), the capacitance of the IO
À1
96 Æ 32 F g , in agreement with those reported for other NiO
10,11,13–15
electrodes. It is expected that the specific capacitance
of the IO electrodes could be further enhanced by optimizing
electrode is maintained up to 80% of that at 5 mV s , versus
only 50% for the NP electrode. The faster charge/discharge
kinetics of the IO electrode is presumably resulting from the
higher electrical conductivity provided by the Ni metal core
and the greater ionic conductivity afforded by the ordered pore
network in the IO structure. This rapid charge/discharge rate
is an attractive feature for high-power supercapacitor applications,
which are essential for fast energy output applications such as
hybrid electric vehicles.
Ni–NiO IO morphological properties (e.g. pore size, NiO shell
1
layer thickness, and annealing temperature).
1
In summary, Ni–NiO core–shell IO electrodes were fabricated
using electrodeposition within a polystyrene bead template. The
XRD and SEM images confirmed that the periodic IO structure
consisted of a Ni metal core and a NiO shell film in well-ordered
hexagonal symmetry. The specific capacitance of the NiO com-
À1
Fig. 4a shows several cycles of representative voltage
profiles of the Ni–NiO core–shell IO electrodes from galvano-
static charge/discharge measurements performed at a constant
ponent of the Ni–NiO IO electrode was about 96 Æ 32 F g
.
The IO electrode showed better rate capability than the
randomly-mixed NiO NP electrode, which is attributed to the
Ni metal core and the ordered pore network in the IO structure.
These results suggest that the Ni–NiO IO architecture appears to
be a promising electrode architecture for supercapacitors.
This work was supported by the U.S. Department of
Energy/National Renewable Energy Laboratory’s Laboratory
Directed Research and Development (LDRD) program under
Contract No. DE-AC36-08GO28308.
À2
current density (0.2 mA cm ). The shapes of the charge/
discharge curves appeared unchanged during the entire
cycling, suggesting that the NiO microstructure is stable
during the repeated charge/discharge cycles. It is noteworthy
À2
that the shapes of voltage profiles cycled at 1 mA cm
(
Fig. S3, ESIw) are essentially the same as the ones shown in
Fig. 4a. There is no significant current–resistance drop,
suggesting that the Ni–NiO IO electrode is suitable for super-
capacitor applications. Cycle performance of the Ni–NiO IO
electrode for the first 1000 cycles, evaluated by plotting the
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Ni–NiO inverse opal electrode. The current density was 0.2 mA cm
.
5
216 Chem. Commun., 2011, 47, 5214–5216